Online retailer Amazon says it had its biggest sales day in the UK as shoppers rushed to buy bargains on Black Friday.
The UK version of the website sold more than 7.4m items while other retailers had problems with web sales due to huge demand.
Retail analysts said online sales were estimated to have passed £1bn on a single day for the first time.
The discount day originated in the US, traditionally kick-starting the Christmas shopping period.
Argos confirmed it had experienced delays on its site after "extremely high levels of visits". John Lewis said its site had been down for a short time in the afternoon.
Last year's Black Friday saw shoppers in the UK fighting over bargains in some stores, leading some retailers to hire extra security staff over the weekend to prevent a repeat.
But analysts said this year many shoppers had chosen the internet over the high street.
Justin Opie, director of the UK's online retail association IMRG said: "The big difference is the industry was much more prepared because it was chaos last year. That was bad for customers and bad for the retailers so everything was much smoother this time."
The BBC's Adam Kirtley, who was at Debenhams' flagship store on Oxford Street in London, said: "The doors opened at 8am and there was a very orderly trickle of people coming in to bargain hunt. No queues, no hassle."
Many big stores have opted for a week long sales period this year and IMRG predict figures will reach over £3bn by Monday - which retailers refer to as Cyber Monday.
The first two Mondays of December tend to be the busiest online shopping days, after browsing over the weekend.
Retail analyst Nick Bubb added: "It looks as if Black Friday spending has been more spread out this year and more weighted to online, but every indication is that the combined event will be bigger than last year overall."
Some retailers in the UK decided not to take part in the event this year. Supermarket chain Asda said it was pulling out due to "shopper fatigue around flash sales".